(1) Field of the Invention
The present invention relates to a rotary wing aircraft having three engines, and to a method of controlling the aircraft.
It should be observed that the term “engine” is used to mean a power unit contributing to providing the aircraft with propulsion and/or lift. For a rotary wing aircraft, the term “engine” is used to designate a power unit that drives rotation of a main gearbox that in turn drives rotation of at least one rotor of the rotary wing.
(2) Description of Related Art
An aircraft is sometimes fitted with an auxiliary power unit (APU). Such an auxiliary power unit may be used for example to generate electricity, to drive hydraulic systems, or indeed to contribute to starting an engine. In contrast, the auxiliary power unit does not drive a main gearbox of a rotor on a rotary wing aircraft.
Consequently, the APU of an aircraft does not constitute an “engine” in the meaning of the invention.
The invention thus comes in the field of power plants for rotary wing aircraft, such as helicopters, for example.
By construction, the level of power that can be delivered by an engine is limited. Under such circumstances, when the power from a single engine is found to be insufficient, manufacturers naturally arrange a plurality of engines on an aircraft.
Installing a larger number of engines on airplanes provides the opportunity to improve the safety of such airplanes. Four-engined airplanes constitute a design that is advantageous for specific missions of crossing oceans or of taking off and landing on short runways.
Nevertheless, in order to reduce complexity and costs, the modern trend is to reduce the number of engines while still performing missions of the same type.
This trend is identical with aircraft having a rotary wing.
For example, three-engined aircraft appeared in the 1960s because of the lack of power of the engines that were available at that time on aircraft presenting heavy weight.
In the heavy category, three-engined rotary wing aircraft are still advantageous for satisfying the increasing requirements of operators in terms of safety. These requirements seek in particular to ensure that a flight is safe regardless of the instant at which an engine failure might occur. Specifically, certain operators desire to be able to continue performing hovering flight in a heavy rotary wing aircraft even if one engine has failed.
Three-engined rotary wing aircraft are fitted with three identical engines. Engines are said to be “identical” when they have identical characteristics for driving a rotary member.
Conversely, engines are referred to as “unequal” when they have different drive characteristics, in particular engines that generate different maximum powers, and/or unequal maximum torques, and/or different maximum speeds of rotation for an outlet shaft. Thus, two unequal engines may correspond respectively to an engine driving an outlet shaft at several tens of thousands of revolutions per minute (rpm), and an engine driving an outlet shaft at less than ten thousand revolutions per minute.
Installing a plurality of engines that are identical was required for the purpose of ensuring responsiveness in the event of one engine failing and also for simplifying the installation and the integration of the engine. Nevertheless, it is always possible to install engines having unequal maximum powers in order to satisfy safety requirements or in order to mitigate the lack of power of engines available on the market.
Nevertheless, the technical challenges that need to be solved have prevented industrialization of any three-engined helicopter architecture that has for example one engine of maximum power different from the maximum power of the others.
In order to design a three-engined rotary wing aircraft having engines that are identical, a manufacturer needs mainly to overcome the technical challenges set out below.
Thus, the engines need to be overdimensioned in order to satisfy safety requirements and be capable of delivering extra power in the event of an engine failing. So-called one-engine inoperative (OH) contingency ratings are implemented. Dimensioning for such excess power levels is very penalizing and incompatible with optimizing an engine in terms of its weight, its cost, its fuel consumption, and its emissions (noise, CO2, . . . ) in particular. It is also very complicated for such engines to be certified since they require additional testing such as specific endurance or “over-temperature” tests.
Furthermore, the engines need to be regulated depending on their utilization.
Aircraft are thus known in which all the engines are regulated as a function of a setpoint that is constant.
Alternatively, and in accordance with present practice, in a multi-engined aircraft, all of the engines are regulated with respect to a setpoint that is variable. For example, the engines are regulated with respect to a setpoint speed of rotation for a free turbine, with this setpoint varying as a function of the power to be delivered or indeed as a function of the density of the surrounding air, in particular.
The engines thus generally co-operate with a control unit, either of the kind known as an engine control unit (ECU), or else of the kind known as a full authority digital engine control (FADEC) unit.
The control unit of any one engine conventionally communicates with the other control units. The engines are thus all regulated as a function of the same variable setpoint.
The setpoint on which engines are regulated varies in particular in order to avoid overspeed of the rotary wing or of an engine.
The power needed on the ground is less that the power needed to take off. As a result, the control units act for example to limit the performance of engines in order to avoid overspeed of the rotary wing.
Conversely, during hovering flight, it is appropriate to avoid overspeed of the engines.
Under such circumstances, integrating three engines may involve control units of considerable size and weight. Compared with a twin-engined aircraft, the number of inputs/outputs in a control unit can be increased significantly, and it is necessary to harmonize communication between the various pieces of equipment.
In addition, the control systems of an aircraft and its engines may excite resonant modes of vibration in a rotary wing aircraft. The complexity of developing torsional stability in the power drive train of the aircraft increases with the number of elements that contribute to the overall drive train, and thus with the number of engines installed.
A good compromise between the reactivity of an engine in response to a command for the pilot and the stability of the aircraft also constitutes a major challenge to overcome. If the engines are very reactive, a rapid command from a pilot runs the risk of exciting a resonant mode of vibration of the aircraft. Given that developing a twin-engined aircraft is difficult, it can be understood that developing an aircraft having three reactive engines is even more difficult.
In order to optimize the operating point of the engines, a power plant having engines of maximum powers that are not equal could be envisaged.
For twin-engined operation, such an installation and its benefits are explained in document WO 2012/059671 A2.
Nevertheless, installing engines having unequal maximum powers involves major technical challenges. Under such circumstances, such a solution would appear to be difficult to implement on a three-engined aircraft.
In particular, the reactivity of such an aircraft can be difficult to optimize.
For example, on a conventional helicopter, the engines are balanced so that the power delivered by each engine is the same.
For an engine with pure proportional regulation, balancing engine power, i.e. “load sharing” is provided by means of a predefined relationship associating the speed of rotation of a gas generator of the engine with the speed of rotation of a rotor providing the helicopter with lift and propulsion. For an engine having proportional-integral regulation that takes account of power parameters (speed of rotation of the gas generator NG, temperature TOT, or indeed torque TRQ) of the other engines in the regulation loop, power balancing is performed by a control unit.
The reactivity of an aircraft having engines that develop equal powers can be better than that of an aircraft having engines that develop unequal powers.
In an aircraft having two engines developing equal powers, each engine delivers half of the required power. If one engine fails, it is then appropriate to accelerate the other engine in order to obtain an increase in power equal to half of the required power.
However, if the engines have unequal maximum powers, the loss of the higher power engine needs to be compensated as quickly as possible by the lower power engine. The lower power engine then needs to be accelerated so as to increase its power by more than half of the power required, given the unbalance.
Furthermore, it can be difficult to control the aircraft under such conditions.
In a conventional helicopter, the engines are regulated so that their free turbines have the same speed of rotation NTL, which speed is variable and proportional to the speed of rotation NR of the lift rotor. As explained above, they are also regulated to deliver identical power.
If the engines have unequal maximum powers, new control logic without power balance would appear to be difficult to develop for the purpose of managing the overall power to be delivered (i.e. the power available from each of the engines, its limits, the power required, etc.).
Furthermore, with engines that do not present identical maximum powers, their speeds of rotation may also be different, and that constitutes another difficulty. Such power management may be even more complex if it is decided to stop one engine in order to improve fuel consumption.
The stability of the aircraft is even more problematic in the presence of three engines of unequal maximum power.
The dynamic behaviors of the engines having different maximum powers can be different. However, information relating to the operation of the engines is exchanged and compared among the controls units. Consequently, unequal engines can be subjected to asymmetrical accelerations giving rise to false alarms concerning transient faults.
The torsional stability of the power transmission drive train of the aircraft can also represent a problem that is difficult to overcome.
Furthermore, it can be difficult to monitor the engines, since unequal engines are difficult to compare with one another.
In addition, if unequal engines are used, an aircraft manufacturer might possibly contact different engine manufacturers for making different engines of a given aircraft.
This might result in difficulties in harmonizing the interfaces between the engines, or between the engines and the remainder of the aircraft.
It can thus be understood that developing a three-engined aircraft ought to be advantageous.
However, implementing three engines that are identical can lead to difficulties in dimensioning the engines and their control units, and also to problems of stability.
Implementing unequal engines appears to be even more difficult, since it gives rise to problems of reactivity, control, stability, monitoring, or indeed integration.
Whatever the variant, making a three-engined aircraft thus gives rise to a variety of difficulties. These difficulties make provision of a three-engined aircraft non-obvious, since a three-engined aircraft is not merely a twin-engined aircraft provided with a third engine.
The technological background includes document U.S. Pat. No. 4,479,619, which proposes a power transmission system for three-engined helicopters.
That solution also proposes the alternative of declutching one out of three engines.
The Super-Frelon helicopter also had three identical engines (without clutching).
Document U.S. Pat. No. 3,963,372 proposes a power management and control solution for the engines of three-engined helicopters. A central unit controls the engines in order to equalize the powers at the outlet from the engines.
In order to mitigate the problems of engines designed so as to be overdimensioned, a power plant having engines with unequal maximum powers for twin-engined aircraft has already been proposed in the past. This applies to document WO 2012/059671 A2, which proposes two engines of unequal maximum powers.
That document WO 2012/059671 A2 deals only with twin-engined aircraft and it does not present solutions to the problems of control or stability.
Document FR 2 933 910 describes a power plant having at least one turboshaft engine and at least one electric motor.
Document US 2009/186320 describes three engines controlled by FADECs that are connected together and to a control member referred to as the “flight control computer (FCC)”.